Modern automobile engines require high octane gasoline for efficient operation. Previously lead (Pb) and oxygenates, such as methyl-t-butyl ether (MTBE) were added to gasoline to increase the octane number. Furthermore, several high octane components normally present in gasoline, such as benzene, aromatics, and olefins, must now be reduced. Obviously, a process for increasing the octane of gasoline without the addition of toxic or environmentally adverse substances would be highly desirable.
For a given carbon number of a light naphtha component, the shortest, most branched isomer tends to have the highest octane number. For example, the branched isomers of hexane, monomethylpentane and dimethylbutane, have octane numbers that are significantly higher than that of n-hexane, with dimethylbutane having the highest research octane number (RON). Likewise, the branched isomer of pentane, methylbutane, has a significantly higher RON than n-pentane. By increasing the proportion of these high octane isomers in the gasoline pool, satisfactory octane numbers may be achieved for gasoline without additional additives.
Two types of octane numbers are currently being used, the motor octane number (MON) determined using ASTM D2700 and the RON determined using ASTM D2699. The two methods both employ the standard Cooperative Fuel Research (CFR) knock-test engine. Sometimes the MON and RON are averaged, (MON+RON)/2, to obtain an octane number. Therefore, when referring to an octane number, it is essential to know which one is being discussed. In this disclosure, unless clearly stated otherwise, octane number will refer to the RON. For comparative purposes, the RON for isomers of hexane and pentane are listed in Table 1.
TABLE 1Research octane number (RON) valuesfor C5 and C6 alkane isomersn-pentane61.7methylbutane92.3n-hexane24.82-methylpentane73.43-methylpentane74.52,2-dimethylbutane91.82,3-dimethylbutane101.0
Gasoline is generally prepared from a number of blend streams, including light naphthas, full range naphthas, heavier naphtha fractions, and heavy gasoline fractions. The gasoline pool typically includes butanes, light straight run, isomerate, FCC cracked products, hydrocracked naphtha, coker gasoline, alkylate, reformate, added ethers, etc. Of these, gasoline blend stocks from the FCC, the reformer and the alkylation unit account for a major portion of the gasoline pool. FCC gasoline, and if present, coker naphtha and pyrolysis gasoline, generally contribute a substantial portion of the pool sulfur.
Gasoline suitable for use as fuel in an automobile engine should have a RON of at least 80, preferably at least 85, and most preferably 90 or above. High performance engines may require a fuel having a RON of about 100. Most gasoline blending streams will have a RON ranging from about 55 to about 95, with the majority falling between about 80 and 90. Obviously, it is desirable to maximize the amount of dimethylbutane in the gasoline pool in order to increase the overall RON. The present invention is directed to this objective.
Hydroisomerization is an important refining process whereby the RON of a refinery's gasoline pool may be increased by converting straight chain normal or singly branched light paraffins into their more branched isomers. The hydroisomerization reaction is controlled by the thermodynamic equilibrium. At higher reaction temperatures the equilibrium shifts towards the lower octane isomers (e.g., from dimethylbutanes via methylpentanes to n-hexane). Since the high octane components (e.g., 2,3-dimethylbutane with a RON of 101.0) are the target products in this process, it is desirable to develop a more active catalyst to perform this reaction at a lower temperature.
In conventional hydroisomerization processes, light normal paraffins are isomerized to their more branched counterparts over a bifunctional catalyst having both acidity (e.g., mordenite or chlorinated amorphous alumina) and hydrogenation/dehydrogenation functionality. Several prior art catalysts have been disclosed to hydroisomerize these lower octane paraffins into the more branched, higher octane isomers (see, for example, U.S. Pat. Nos. 7,029,572 to Maesen et al. and 6,140,547 to Lin et al.).
U.S. Pat. Nos. 5,082,988 and 5,166,112 both to Holtermann each disclose a process and a catalyst for isomerizing normal and slightly branched C4 to C7 hydrocarbons, the catalyst comprising a Group VIII metal on Beta zeolite. Chica et al. (J. Catalysis 187, 167-176 (1999)) compared the hydroisomerization of light paraffins by nanocrystalline Beta zeolite and various other materials including zeolite SSZ-33. U.S. Pat. Nos. 4,910,006 and 5,007,997 both to Zones et al. disclose the isomerization of C4 to C7 hydrocarbons using zeolite SSZ-26, including the isomerization of pure hexane to give a product with a 2,3-dimethylbutane:2,2-dimethylbutane ratio of about 0.55. U.S. Pat. Nos. 4,963,337 and 5,120,425 both to Zones et al. disclose the isomerization of C4 to C7 hydrocarbons using zeolite SSZ-33. U.S. Pat. No. 5,233,121 to Modica discloses a process for isomerizing a light paraffinic naphtha feedstock using a catalyst comprising a zeolite Beta component.
There is a need for new and improved hydrocarbon hydroisomerization catalysts and processes that provide high selectivity for producing high octane isomers of light paraffins, wherein the catalysts are also highly active, environmentally benign, and readily regenerable.